Magnetic robot crawlers configured to traverse ferrous surfaces such as construction steel, tanks, piping, pier pilings, or hull of a ship have been designed. Some crawlers use electromagnets (see for example U.S. Pat. No. 4,890,567); others use permanent magnets as disclosed in U.S. Pat. Nos. 3,682,265; 3,777,834; 5,285,601; and 5,894,901 all incorporated herein by this reference.
U.S. Pat. No. 5,285,601 shows a robot with permanent magnet track treads. U.S. Pat. No. 5,894,901 discloses a complex design with circulating permanent magnets in the track. U.S. Pat. No. 3,682,265 shows a v-belt track below fixed permanent magnets.
Some structures include discontinuities such as weld beads up to 0.5 inches tall as well as other obstacles. If a ferrous surface is submerged and exposed to currents of 15 knots, in one example, the drag and lifting forces on the robot can be substantial. If the robot drives over a discontinuity, it may pitch outwardly from the surface and can be lifted off the surface due to the drag and/or lift forces resulting in a loss of the robot especially if it is not tethered to the surface.
Since autonomous operation is desirable and since such a robot can be expensive, it is desirable to avoid the loss of the robot. If rotating magnetic treads are used, energy must be used to lift the treads off the hull during revolution of the tracks which can affect battery life in the case of an autonomous battery powered robot.